Neonates are incapable of maintaining their own body temperature during the first few weeks of life. Skin perfusion rates are very high and the infant loses heat rapidly. Thermal management is critical, requiring an accurate, fast, noninvasive method of core temperature measurement.
Rectal temperature has long been considered to be the standard indicator of neonate core temperature. However, since temperature measurements from different locations on a neonate""s skin are sufficiently uniform as to be relatively interchangeable with one another, the clinician may select the most noninvasive and convenient site at which to measure temperature. Due to its inherent safety and long established efficacy, axilla (underarm) is the most recommended site for neonates. Unfortunately, conventional thermometers such as glass/mercury, electronic and paper strip thermometers require up to several minutes to obtain an accurate axillary reading.
In recent years, infrared thermometers have come into wide use for detection of temperature of adults. For core temperature readings, infrared thermometers which are adapted to be inserted into the patient""s ear have been extremely successful.
Infrared ear thermometry has not found such high acceptance for use with neonates. Neonates have a very high moisture level in their ear canals, due to the presence of vernix and residual amniotic fluid, resulting in low tympanic temperatures because of the associative evaporative cooling. In addition, environmental uncertainties, such as radiant heaters and warming pads can significantly influence the air temperature. Further, clinicians are less inclined to position the tip of an infrared thermometer in the ear of a small neonate.
An infrared thermometer designed for axillary temperature measurements is presented in U.S. patent application Ser. No. 08/469,484. In that device, an infrared detector probe extends from a temperature display housing and may easily slide into the axilla to lightly touch the apex of the axilla and provide an accurate infrared temperature reading in as little as one-half second.
The axillary infrared thermometer has found great utility not only with neonates but as a screening tool in general and especially for small children where conventional temperature measurements such as a thermometer under the tongue or a rectal thermometer are difficult. The axillary measurement is particularly accurate with children where there is a lack of axillary hair and perspiration.
The present invention has particular applicability to an axillary infrared thermometer which is suitable for nonclinical use but which retains the accuracy required for clinical use.
In accordance with one aspect of the invention, the thermometer comprises an extended housing forming a wand which has a handle portion at one end and a radiation detection portion at the other end. The wand configuration allows the device to be passed through the sleeve of the patient to the axilla to minimize exposure of the patient to the surrounding environment. Further, the housing surface is formed of a low thermal-conductivity material to minimize cooling of the patient with contact against the skin. The radiation detection portion of the housing comprises an axially directed window positioned within a cup. The window passes radiation to an infrared radiation sensor within the housing from a field of view substantially less than the area of the cup opening.
In accordance with another aspect of the invention, the handle portion of a radiation detector housing, preferably configured as a wand, extends along an axis generally parallel to the viewing axis but offset from the viewing axis.
In accordance with another aspect of the invention, the cup is designed to conduct heat from the target area in order to match the heat loss from the target area when the radiation detector is not in place, thereby minimizing changes in target temperature.
In accordance with another aspect of the invention, the radiation sensor is mounted within a large thermal mass within the low conductivity housing to provide an RC time constant for change in temperature of the radiation sensor, with change in temperature to which the housing is exposed, of at least 5 minutes and preferably 25 minutes.
In accordance with another aspect of the invention, a transparent plastic film is positioned over the cup to minimize the effects of evaporation from the target surface. The thickness of the film is at least 1.1 mils. and is preferably 1.5 mils.
In a preferred configuration, the radiation sensor is mounted within a can for viewing a target through an infrared transparent window on the can and through an aperture at the base of an emissivity compensating cup. The can is mounted within a bore in a heat sink with thermal coupling to the heat sink through a rear flange. The flange may be pressed against a shoulder by an elastomer such as an o-ring. A rear cap fit to the heat sink presses against the elastomer to press the can against the shoulder. Preferably, the cap is an externally threaded plug which fits within the bore. The cap has an opening therein through which electrical leads to the radiation sensor pass.
In accordance with another aspect of the invention, a novel detection circuit is provided. A radiation sensor provides an output as a function of difference between target temperature and sensor temperature over a design range of target temperatures and a design range of sensor temperatures. An amplifier amplifies the sensor output and an analog-to-digital converter generates a multibit digital output from the amplified output over a voltage range of the amplified sensor output. A reference to the radiation sensor and amplifier circuit is variable to provide high analog-to-digital converter resolution, over the design ranges of target and sensor temperatures, with sensor temperatures either above or below target temperature. The resolution is greater than would be obtained with a fixed reference over fall design ranges of target and sensor temperatures. The variable reference provides a variable offset which may be subtracted out in digital processing circuitry prior to digital computation of target temperature.
In the preferred detection circuit, the reference is variable to offset the amplified output by an offset level approximating sensor temperature. The amplified sensor output then approximates target temperature. In an alternative embodiment, the reference is set at one of two levels depending on whether target temperature is above or below sensor temperature. In one implementation of that embodiment, the reference is set to one of those two levels only after the amplified output has exceeded the analog-to-digital converter range using an intermediate reference.
In each embodiment of the detector circuit, the reference level is preferably set while also compensating for amplifier offset. The radiation sensor is isolated from the amplifier and the amplified output is varied either to approximate the sensor temperature or to set the amplifier output at one of three levels corresponding to the two reference levels. Preferably, a resistor which balances the resistance of the radiation sensor is also isolated from the amplifier during the offset calibration.